"Today all of us are sensualists," Nietzsche writes in Book Five of The Gay Science.¹ This striking assertion provides a signal for a set of problems concerning knowledge or, more particularly, the relation between knowledge and life. Nietzsche had been concerned with these questions for a long time, but only after Thus Spake Zarathustra did he define them in the ways that provide the theme of this discussion. As his thinking found its own path more and more, Nietzsche moved decisively away from the pessimism and romanticism which had earlier influenced him, and gained a new appreciation of the philosophers of the French Enlightenment, even describing their sensualism and hedonism as the "best inheritance" available to his own century.² Hence, for example, his suggestion that the credit for Stendhal, in his opinion the greatest French writer of the nineteenth century, must go to "the best, most rigorous philosophical school in Europe, that of Condillac and Destutt de Tracy."³ The term 'sensualist," as used here, refers in the first instance to these thinkers who, proceeding from the empiricism of Locke, attempted to derive all ideas from elementary sensations. (Hence the alternative expression, 'sensationalism.") It does not imply a preoccupation with sensual pleasure, although the English word may be used most often in that sense.⁴ On the other hand, it can hardly be denied that any moral philosophy which finds its basic evidence in sensation will be naturally inclined towards hedonism, or at least utilitarianism. Less evident, but equally seen in these French writers, is the affinity between sensualism and a materialist interpretation of human nature.